Field of the Disclosure
The present application relates to an organic light emitting element. More particularly, the present application relates to an organic light emitting element adapted to enhance luminous efficiency in a structure including at least two organic light emission layers.
Description of the Related Art
The display field for visually representing an electrical information signal has been rapidly developed with the spread of substantial information society. In accordance therewith, a variety of flat panel display devices with features, such as slimness, light weight and low power consumption have been developed. Also, the flat panel display devices have been rapidly replacing the existing cathode ray tubes (CRTs).
As examples of the flat panel display devices, liquid crystal display (LCD) devices, organic light emitting display (OLED) devices, electrophoretic display (electric paper display (EPD)) devices, plasma display panel device (PDPs), field emission display (FED) devices, electroluminescence display devices (ELDs), elector-wetting display (EWD) devices, and so on can be introduced. Such flat panel display devices commonly include a flat display panel, which realize an image, as a necessary component. The flat display panel is configured with a pair of combined substrates which face each other with having an inherent light emitting or polarizing material layer therebetween.
The organic light emitting display device corresponding to one of the flat panel display devices includes an organic light emitting element which is a self-illuminating element. As such, the organic light emitting display devices do not need a separate light source which is used in a liquid crystal display device. In accordance therewith, organic light emitting display devices are lighter and thinner. Also, organic light emitting display devices have features of wider viewing angle, superior contrast, and lower power consumption than those of the liquid crystal display devices. Moreover, organic light emitting display devices can be driven by a low direct-current voltage and provide a high speed response. Furthermore, organic light emitting display devices can resist external impacts and be used in a wide temperature range because of having solid components.
The organic light emitting element applies an electric field to an organic light emission layer and forces the organic light emission layer to emit light. The organic light emission layer is formed from an organic material and on a glass substrate in such a manner as to be disposed between an anode and a cathode. The anode is formed from ITO (indium-tin-oxide) or others, and the cathode is formed from aluminum or others. In detail, if a voltage is applied between the anode and the cathode of the organic light emitting element, electric-holes injected from the anode and electrons injected from the cathode move toward the organic light emission layer and are recombined with each other in the organic light emission layer, thereby forming excitons. Light can be emitted by which the excitons are transitioned from an excited state to a ground state.
Such an organic light emitting display device can be fabricated in a single stack structure or a multi-stack structure including a plurality of stacked organic light emitting elements. The multi-stack structure can provide a high efficiency when white organic light emitting elements are included. However, as the number of stacks increases, a driving voltage must be increased.
Alternatively, a structure of including a plurality of light emission layers within a single stack can be applied to the organic light emitting display device. In this case, light emitted in a light emission layer adjacent to the cathode which is used to inject the electrons must be absorbed into another light emission layer adjacent to the anode which is used to inject the electric-holes. More specifically, in order to enhance luminous efficiency, each of the light emission layers includes a host. The host of a light emission layer adjacent to the anode absorbs light emitted from another light emission layer adjacent to the cathode. Due to this, an external luminous efficiency of the light emission layer adjacent to the cathode must deteriorate.